This invention relates to a structure for mounting a thermistor with positive resistance-to-temperature characteristic. More particularly, this invention relates to thermistors with improved voltage resistance.
Thermistors with positive resistance-to-temperature characteristics (hereinafter referred to simply as the positive characteristic) are used, for example, in overcurrent protection circuits in the telecommunication field. FIGS. 17, 18 and 19 show a prior art structure for mounting a positive characteristic thermistor 1 for such a use, FIG. 17 being a plan view of a portion of a hybrid IC having the thermistor 1 mounted thereto, FIG. 18 being a sectional view taken along line XVIII--XVIII in FIG. 17 and FIG. 19 being a sectional view taken along line XIX--XIX in FIG. 17.
As shown in FIGS. 17-19, the positive characteristic thermistor 1 has electrodes 2 and 3 formed respectively on its mutually opposite main surfaces. This thermistor 1 is mounted to a substrate 4 made, for example, of a ceramic. The substrate 4 has conductive members which are each electrically connected to a corresponding one of electrodes 2 and 3. According to this prior art example, the conductive members include conductive lands 5 and 6 formed on the substrate 4, a connecting member 8 to be connected to the conductive land 6 through solder 7, a solder part 9 provided so as to electrically connect the conductive land 5 and the electrode 2, and another solder part 10 for electrically connecting the connecting member 8 and the electrode 3. Lead terminals 11 and 12 are also attached to the substrate 4 for electrically connecting to the conductive lands 5 and 6, respectively.
The resistance against voltage by a thermistor 1 thus structured sometimes deteriorates, depending upon how it is mounted, compared to before it is mounted. This is because the escape of heat from the ceramic substrate 4 is relatively high such that, when the electric energy applied to the thermistor 1 is converted into heat, a large portion of this heat energy escapes through the solder parts 7, 9 and 10, as well as the connecting member 8, thereby increasing the heat flow from the thermistor 1 and destroying its balance.